Internal combustion engine protection circuit

ABSTRACT

Four sensor input circuits continuously and simultaneously monitor liquid coolant temperature, liquid coolant level, engine speed and engine oil pressure. When predetermined conditions are reached in either the liquid coolant temperature or level input circuits an alarm signal is provided directly to a trip circuit which simultaneously operates one alarm which is a continuous visual or audible alarm and a second alarm which shuts down the engine and is itself de-energized after a predetermined time interval. The liquid coolant temperature and level input circuits are fail safe, indicating an alarm in the event of a short or open circuit in either sensor. The signal from the speed input circuit is compared to a predetermined value representative of maximum desired engine speed in a separate high speed trip circuit to trip the said alarms in the event of an overspeed. The output signals from the speed input circuit and the pressure input circuit are each variable DC signals which are adjustable in relation to each other and which are compared in a comparator to generate an alarm signal to the trip circuit whenever the oil pressure falls below a predetermined value for a given engine speed. Notwithstanding provision of a variable battery power supply, all units are operated at a predetermined regulated voltage level. Also provided are circuits for protecting against false trips and for protecting electrical units from accidental reverse application of supply power.

United States Patent McBride, Jr. et al. July 1, 1975 INTERNAL COMBUSTION ENGINE [57] ABSTRACT PROTECTlON CIRCUIT Four sensor input circuits continuously and simulta- [75] Inventors: Lyle E. McBride, Jr., Norton; neously monitor liquid coolant temperature, liquid William W. Bowman, North Easton, coolant level, engine speed and engine oil pressure.

both of Mass. When predetermined conditions are reached in either [73] Assignee: Texas Instruments Incorporated, the liquid goolarit tempiarature. or level inplit l i Dallas Tex. an alarm signal is provided directly to a trip circuit which simultaneously operates one alarm which is a [22] Filed: Dec. 20, 1973 continuous visual or audible alarm and a second alarm which shuts down the engine and is itself de-ener ized [21] Appl' 426850 after a predetermined time interval. The liquid co lant temperature and level input circuits are fail safe, indi- [52] U.S. Cl. 340/420; 340/52 F; 340/57; eating an alarm in the event of a short or open circuit 340/60; 180/105 E in either sensor. The signal from the speed input cir- [51] Int. Cl. G08b 23/00 cuit is compared to a predetermined value representa- [58] Field of Search 340/52 F, 53, 57, 60, 27 B, tive of maximum desired engine speed in a separate 340/420; 180/82 R, 103, 105 E; 317/13, 36 high speed trip circuit to trip the said alarms in the R, 40, 141 event of an overspeed. The output signals from the speed input circuit and the pressure input circuit are [56] References Cited each variable DC signals which are adjustable in rela- UNITED STATES PATENTS tion to each other and which are compared in a com 3 686 668 8/1972 Durkee 340/420 parator to generate an alarm Signal to the trip Circuit 3723:964 3/1973 Lace I h 340/52 F whenever the oil pressure falls below a predetermined 772,642 11/1973 Schlorke v I 340/52 p value for a given engine speed. Notwithstanding provi- 3,776,357 12 1973 Arai et al.... 180 82 R s of a variable battery POWer Supply, all units are 3,819,004 6/1974 Adde 180/82 R operated at a predetermined regulated voltage level.

Primary Examiner-Thomas B. Habecker Attorney, Agent, or FirmJames P. McAndrews; John A. Haug; Russell E. Baumann Also provided are circuits for protecting against false trips and for protecting electrical units from accidental reverse application of supply power.

28 Claims, 6 Drawing Figures SHEET LS5 x kwmw RS UQQ DSMN INTERNAL COMBUSTION ENGINE PROTECTION CIRCUIT BACKGROUND OF THE INVENTION 1. Field Of The Invention This invention relates to an electronic protection sys tem for an internal combustion engine, and, more particularly to a protection circuit capable of sensing multiple alarm conditions and simultaneously operating multiple alarm circuits continuously or momentarily or both.

2. Description Of The Prior Art Electronic protection circuits to monitor operating parameters of equipment have been used for a variety of purposes with varying degrees of success.

Furnace control circuits are known which use a time dependent thermistor responsive to temperature in a circuit which shuts off the fuel supply to the furnace if fuel in the furnace is not ignited by the time the thermistor reaches a predetermined temperature. See, for example, U.S. Pat. No. 3,549,088.

Various protection circuits for electrical loads are also known. One such circuit uses a PTC or NTC thermistor to sense an overheat condition in the electric motor and generate a shut down signal to a normally conducting semiconductor to de-energize the motor. See, for example, U.S. Pat. No. 3,569,781. Similarly, another circuit monitors the actual current drawn by an electric motor during start-up and compares the result with a reference value, the two inputs being in the form of variable DC signals applied to the input terminals of a differential amplifier, to de-energize the motor and prevent it from being operated in an undersired mode thereby limiting the steady state current carrying ca pacity needed in semi-conductors used in the circuit. See, for example, U.S. Pat. No. 3,609,461. Another such circuit uses multiple PTC sensors in a circuit to sense high impedance changes in a sensor resistance and operate a double alarm indicator such as a pop-up flag which requires manual reset and an overload light. The latter circuit includes protection against a short circuit in the sensors and a lockout feature preventing reapplication of power unless the entire circuit is deenergized. See, for example, commonly owned pending application Ser. No. 316,194 filed Dec. 18, 1972.

As part of an effort to increase safety devices in consumer products, particularly automobiles, a protection circuit has been developed which monitors the brake fluid level in the master cylinder of a vehicle and actuates a lamp when the liquid level is too low. This circuit uses a temperature sensitive sensor which has a low resistance in liquid and a high resistance in air. See, for example, U.S. Pat. No. 3,760,352. See also, U.S. Pat. No. 3,766,395.

A protection circuit is known which, after a predetermined delay following start-up of an air conditioning compressor, shuts down the compressor if a pressure switch fails to close indicating that the oil pressure has failed to reach a minimum operating level. The circuit includes simultaneous continuous monitoring of the temperature of the windings of the compressor with shut down being triggered upon sensing an overheat condition. See, for example, U.S. Pat. No. 3,753,043.

The above devices, though useful in their own fields, have not found application in continuous monitoring of internal combustion engine primary operating parame ters for initiating automatic action upon the detection of an unsafe condition. Normally the coolant temperature, engine speed and oil pressure of an internal combustion engine have in the past been indicated on a gauge, or, by an on-off light which signals a safe operating level. Liquid coolant level has generally not been monitored in the past. These indicators require observation of the indicated condition being measured, recognition of an unsafe condition and then corrective action by the driver or operator. 1n unattended installations, none of the aforesaid functions can be performed. In attended installations frequently the human element fails to act, or fails to act quickly enough to prevent damage to the equipment or injury to person nel. For example, though oil pressure and engine speed are both indicated on a gauge and observed, the operator may be unaware that the oil pressure is unsafe for the particular speed at which the equipment is operating and would fail to recognize that the equipment should be shut down.

These shortcomings are overcome in the present invention. There is provided a protection system for si multaneously and continuously monitoring all four primary engine parameters coolant temperature, coolant level, engine speed and oil pressure as a function of engine speed and for automatically indicating alarm conditions while automatically taking action to protect the equipment.

SUMMARY OF THE lNVENTlON A speed sensor input circuit generates a variable DC signal which is used as one input to a differential speedpressure comparator. The other input to the comparator is a variable DC signal representative of engine oil pressure. The latter signal is adjusted in relation to the first signal so that whenever the oil pressure signal falls below the speed signal, the comparator senses the difference and generates an alarm signal representing insufficient oil pressure for a given speed. The alarm signal is first delayed to filter out false alarms and is then sensed by a biased differential amplifier which provides an output signal which simultaneously operates a continuous visual or audible alarm, a solenoid means for shutting down the engine and an alarm shut down circuit to deenergize the solenoid after a predetermined time interval. A high speed trip circuit compares the speed input signal to a predetermined value representative of the maximum desired speed and provides an alarm signal. Additional input circuits continuously monitor liquid coolant temperature and level and generate an alarm signal when excessive temperature or low level conditions are reached. Steering diodes prevent interference between any of the aforesaid alarms, and any of the alarms will trip the differential amplifier and operate both the audible/visual alarm and the solenoid. Provision is made to signal an alarm upon a short or open circuit in the sensors of the coolant temperature and level input circuits. The fail safe circuit of the level input circuit has a primary channel which operates by lowering the voltage level at the positive terminal of an operational amplifier below two volts to cause the output of the amplifier to go high generating an alarm signal which reaches the aforesaid biased differential amplifier prior to a nearly simultaneous alarm signal generated in the secondary or back-up channel of the fail safe circuit.

Accordingly, it is an object of the present invention to operate a low power, continuous audible or visual alarm upon the sensing of any one of four predetermined alarm conditions which are:

l. The oil pressure for any given speed falls below a predetermined pressure for a given engine speed.

2. The speed of the engine exceeds a predetermined RPM.

3. The temperature of the liquid coolant exceeds a predetermined temperature.

4. The level of the liquid coolant in the engine drops below a predetermined level.

Additional objects of the present invention are as follows:

1. To provide affirmative automatic action in the form of shutting down the engine upon the occurrence of any of the four aforesaid alarm conditions.

2. To provide for turning off of the high power action mode alarm to avoid draining supply power on unattended equipment installations.

3. To provide for avoidance of false alarms.

4. To provide for failsafe protection of the sensor input circuits. Other objects will become apparent from the description of the drawings and the preferred embodiments which are set forth below.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the protection circuit of the present invention.

FIGS. 2A and 2B are circuit diagrams of one embodiment of the present invention.

FIG. 3 is a chart showing the output voltage of the pressure input circuit.

FIG. 4 is a chart showing the output voltage of the speed input circuit.

FIG. 5 is a chart showing the relation of pressure to speed which causes the delayed pressure speed comparator to trip.

DESCRIPTION OF THE PREFERRED EMBODIMENT Overall Operation Referring first to FIG. 1 there is shown a block diagram of the four sensor protection system of the present invention. The power supply 1 includes a replaceable, externally mounted, dropping resistor which permits operation of the entire circuit with either a 12, 24 or 36 volt supply battery. Input circuit 2 includes the sensor and associated circuitry for sensing high temperature of the coolant liquid and provides an alarm signal to the trip circuit whenever the temperature exceeds a predetermined temperature. Input circuit 3 includes the sensor and associated circuitry for sensing the low level of the coolant liquid and provides an alarm signal to the trip circuit 10 whenever the liquid level falls below a predetermined level. Open-short sensor circuits 6 provide fail safe protection against open or short circuits in the temperature and level sensors of input circuits 2, 3. Input circuit 4 includes the sensor and associated circuitry for sensing the speed of the engine and generating a variable DC signal which is representative of the speed of the engine. This signal is read directly on a meter in circuit 7. The speed signal generated by input circuit 4 is also provided to a high speed trip circuit 8 which provides an alarm signal to the trip circuit 10 when the speed of the engine exceeds a predetermined value. The output signal of circuit 4 is also provided as the reference input signal to the pressure speed comparator circuit 9. Input circuit 5 includes the sensor and associated circuitry for generating an adjustable, variable DC signal which is representative of the actual oil pressure of the engine. The signal is provided as an input signal to the pressure-speed comparator circuit 9 which generates an alarm signal to trip circuit I0 whenever the actual oil pressure for a given engine speed falls below a predetermined value. Trip circuit l0 prevents interference between the alarm signals from the four input circuits and, in a genuine alarm condition, simultaneously operates both alarm circuits 11 and 12 and alarm output shutdown circuit 13. The first alarm output circuit 11 is a continuous audible or visual alarm. The second alarm output circuit 12 is a momentary alarm which shuts down the engine. After a few seconds of operation alarm circuit 12 is shut down by alarm shutdown circuit 13. The protection system further includes a 10v regulator circuit for all units requiring supply power, circuits for protection against false alarms being generated in circuits 8 and 9 and protection against accidental reverse application of supply power.

High Coolant Temperature Alarm Circuit The purpose of this circuit is to sense an overtemperature condition in the liquid coolant and generate an alarm signal. This circuit includes thermistor T,, resistors R and R and diode D Refer to FIG. 2B.

The characteristics of thermistor T, which is a PTC sensor are well known in the art. Any known sensing device capable of presenting a sharply increased electrical resistance in response to an increase in temperature may be used.

Thermistor T, and resistor R are connected across a special 10 volt power supply provided by transistor Q and Zener diode Z as indicated at l in FIG. 1 and as described more fully below, and perform as a voltage divider. The junction between T, and R is connected to the positive terminal of differential comparator amplifier A, through R and D When a predetermined temperature is exceeded and sensed by T,, the voltage at A goes high and is reflected proportionately to D and at the positive terminal of A, in the trip circuit 10 as an alarm signal, as described in greater detail below.

An open-short sensor circuit is included as a fail safe protective device for the high coolant temperature alarm circuit. The fail safe circuit functions to provide an alarm signal in the event T, is short circuited. The shorted sensor circuit includes resistors R 37 and transistor 0, and diodes D, and D During normal operation transistor O is turned on by 10 volt regulator 14 indicated in FIG. 1, the current flow being from supply through R the base-emitter junction of Q and R to ground. The current flow from the collector to emitter of O is limited by R The voltage level at the collector of O in normal operation, as determined by the constants of the circuit, is insufficient to constitute an alarm signal to comparator A, of the trip circuit. If T, short circuits, however, the voltage level at A goes to ground. Current flow is then through R R D and then to ground. The voltage level at the base of goes down and O is turned off causing the voltage at the collector of O to go high to supply voltage level which is reflected proportionately to D and then at the positive terminal of A, in the trip circuit as an alarm signal as described in greater detail below. R clamps the voltage level at the emitter of Q, at a stable level so that when the base of O goes low due to a short circuit,

turns off. If the high temperature coolant sensor opens, voltage at point A goes high and trips comparator A as in the normal mode of operation.

Low Coolant Level Alarm Circuit The purpose of this circuit is to sense a low liquid level condition in the coolant system and to generate an alarm signal in response thereto. This circuit includes level sensor L resistors R R R R comparator A capacitor C and diode D Refer to FIG. 2B.

The low coolant level alarm circuit works similarly. to the high coolant temperature alarm circuit, and although the level sensor is more sensitive to voltage and operates at a higher power level, it; utilizes the special voltage supply indicated at l of FIG. 1. The special voltage is provided bytransistor Q and Zener diodez as is more fully described below.

Level sensor L may be any known sensing device which is capable of presenting an increased electrical resistance in response to a decrease in coolant liquid level. Such sensing devices are usually cooled by a liquid and electrically heated in air to present an increased electrical resistance.* Parallel resistors R and R are connected in series with sensor L and form a voltage divider the junction of which is connected to the positive terminal of amplifierA through a voltage divider comprising R and R The reference level of amplifier A is set by voltage divider R R which is the same voltage divider setting the reference level for differential comparator A as more fully described below. C is a noise filter. The separate amplifier A is needed in order to regulate the signal generated by the level sensor and provide an accurate alarm signal, The output signal from amplifier A drives comparator A through steering diode D A similar type temperature sensitive sensor known as an NTC sensor presents a decrease in electrical resistance in response to increased temperature. Given applicants' disclosure, it would be obvious to substitute such a sensor in a circuit to generate an alarm signal in response to either an increase in:coolant-temperature or a decrease in coolant level In operation, when the level of the coolant liquid in the engine goes below a certain predetermined level, L heats up and the voltage level at point C goes high raising the voltage level at the positive terminal of comparator A A senses the alarm condition and generates an output alarm signal which is reflected proportionately to diode D and at the positive terminal of comparator A of the trip circuit, as more fully described below.

The voltage supply portion of the low coolant level alarm circuit receives power from pin E and functions to regulate the supply voltage applied to L and in; cludes transistor Q diode D Zener diode Z3, resistor R and capacitor C In operation Q, is normally con ducting and its output voltage at the emitter is regulated at 10v by Zener diode Z which is set at about 11v. C filters out transient noises. R sets the level of the base current to Q v Diode D protects transistor Q againstacci'dental'reverse connection of the supply battery.

The level sensing alarm circuit also includes a fail safe circuit which functions to provide an alarm signal in the event I; is short circuited. The fail safe circuits include a primary fail safe circuit and back-up fail safe circuit. i 1 i The primary fail safe circuit includes the same elements as the normal alarm circuit, namely li R and A C and D When L approaches a'short circuit, the voltage level at point C "goestoward ground,

lowering the voltage level at the positive terminal of A to=about .7, volts, well below the voltage level at the negative terminal of A One would normally expect the output signal of operational amplifier A to go low. Such is normally the result when the voltage level at the positive terminal goes below the voltage level at the negative terminal, except for an unusual characteristic of operational amplifiers which is taken advantage of in the present circuit. That is, when the voltage level at the plus terminal of an operational amplifier goes below common mode voltage, the voltage output of the amplifier goes high, even though the voltage level atthe negative terminal exceeds the voltage level at the positive terminal. Normally operational amplifiers are operated above the 2 volt level specifically to avoid this unusual -result.'Here, however, this abnormality is used to advantage. The constants of the circuit are selected so that upon a short circuit of L,, the voltage level applied to the positive terminal of A goes below common mode voltage, causing the output of A to go high generating'an alarm signal which is reflected proportionately to D and the positive terminal of amplifier A in the trip circuit, as more fully described below.

The backup fail safe circuit includes Zener diode Z which normally is not conducting. If L short circuits, the voltage at point C goes to ground, placing Z in parallel with Z -and lowering the voltage level at the base of Q The output voltage of Q at the emitter is thereby loweredsufficiently that the voltage level at the junc' tion of voltage divider R R and at the negative terminal of- A goes down sufficiently to constitute an alarm signal.

Furthermore, the circuit constants are selected such that the alarm signalprovided by the primary fail safe circuit through A and D to the positive terminal of A arrives prior to the alarm signal provided by the secondary fail safe circuit througlh Z Q and R to-the negative terminal of A By providing primary and back-up alarm signals, additional reliability is built into the circuit, since A will still trip in the event L shorts, even if amplifier A fails. Z is selected to conduct at about 4v as compared to the llv level of 2;, and performs an additional function of limiting current flow from O through R to ground during a short circuit of L which'permits using a simpler heatsink for Q and reduces construction costs. If level sensor L opens, voltage at point-C goes high and trips A and then A as in the normal mode of operation.

Overspeed Alarm Circuit The purpose of this circuit is to sense an overspeed condition of the engine and generate an alarm signal. The overspeed alarm circuit includes the speed input circuit, the speed meter circuit, and the high speed trip circuit. r

The speed input circuit includes speed sensor R resistors R R R R R R R R R R R and R 5, capacitors C,, C C C and C hi gain amplifiers A and A integrated circuit 1C and Zener diode Z Refer to FIG. 2A. I

The operation of the speed circuit is as follows:

Speed sensor R is a wound core which is physically placed adjacent to the ring gears of the engine flywheel. As each 'tooth of the ring gear passes the wound core of R the magnetic flux in the core changes generating an AC signal, the frequency of which is propor tional to the speed of the engine. Zener diode Z is coupled between the junction of R and Rmand ground and functions to limit the amplitude of the AC signal generated by R in the event of a high voltage spike. Thus, capacitor C a DC blocking capacitor. is presented with a variable frequency AC signal, the frequency of the signal being proportional to the speed of the engine. C is coupled to the positive terminal of high gain amplifier A The negative terminal of amplifier A is set by voltage divider R and R The positive terminal of A is connected in series with the negative terminal through R; and R R is much larger than R,,. When capacitor C is not conducting, both terminals of amplifier A are at the same voltage level. When capacitor C conducts, most of the signal goes through R to the plus terminal of amplifier A generating a large increase in the output signal of the amplifier. Thus, the output of A is an amplified signal closely approximating a square wave and having a constant amplitude and variable frequency which follows the AC signal generated by R The output of A is differentiated in R and C and applied to the negative terminal of hi gain amplifier A, which performs in a manner similar to amplifier A Voltage divider R and R holds the negative terminal of amplifier A at an operating level. The output of amplifier A is a square wave AC signal, i.e., a constant amplitude, variable frequency signal, the frequency being proportional to the speed of the engine. This signal is fed into integrated circuit IC at pin 2. IC is a monostable multivibrator well known in the art which converts the input signal to a constant pulse width, constant amplitude AC signal, the number of pulses per unit of time being a function of the speed of the engine. The output of IC,, at pin 3 is integrated in RC circuit R C the output of which is a variable DC voltage proportional to the speed of the engine.

This variable DC output signal is applied to three locations. First it is read at a meter at terminal D which comprises the speed meter circuit.

Second, the same signal is applied to a high speed trip circuit the purpose of which is to sense an overspeed condition, such as would be caused by a governor failure, and to generate an alarm signal in response thereto. The high speed trip circuit includes resistors R R R R and R comparator amplifier A capacitor C and diode D In operation the variable DC output signal from IC is connected through R to the positive terminal of comparator A The reference voltage level at the negative terminal of comparator A is set by voltage divider R R and R R is an adjustable resistor so that a predetermined level representative of a selected maximum speed can be set at the negative terminal of amplifier A When the voltage at the positive terminal exceeds the reference voltage, the output of A goes high and is reflected proportionately to diode D and at the positive terminal of A, in trip circuit as an alarm signal, as described more fully below. RC circuit R C functions as a small delay circuit for the high speed trip circuit and filters out noise which might generate a false alarm.

Third, the variable DC voltage output signal from the speed circuit is applied through R as a reference input signal to the positive terminal of the pressure-speed comparator A as described in greater detail below.

Low Oil Pressure Alarm Circuit The low oil pressure alarm circuit includes the presthe variation in oil pressure of the engine as a function of engine speed. This circuit comprises a pressure sensor P resistors R R R R R R R R and R and amplifier A Resistors R and R are variable resistors. Refer to FIG. 2A.

P is a sensor whose characteristics are known in the art. P senses pressure changes and generates a variable DC voltage in response thereto. P is located in the oil line of the engine and is powered by 10 volt regulator 14 as indicated in FIG. I.

The output of P is coupled through R and one branch ofa bridge network comprising R R R and R to the positive terminal of amplifier A The level of the reference voltage applied to the negative terminal of amplifier A, is set by the other branch of the bridge network comprising R R and R When the voltage level of the variable DC signal from P exceeds the reference voltage, amplifier A senses the difference and generates a variable DC voltage output signal which is proportional thereto. This variable DC output signal from A is applied as the input signal to the negative terminal of pressure-speed comparator A and is a signal representative of the oil pressure of the engine at any given moment.

As described above, the variable DC output signal from the speed circuit is applied as the input signal to the positive terminal of pressure-speed comparator A and is a signal representative of the speed of the engine at any given moment.

Thus, pressure-speed comparator A, has two variable DC voltages applied to its input terminals. Comparator A senses any difference in these DC voltages. So long as the variable DC voltage applied to the negative terminal of A representative of actual oil pressure is at any instant equal to or greater than the variable DC voltage applied to the positive terminal of A representative of engine speed, no alarm signal is sensed by A and oil pressure for the given speed is satisfactory for continual operation. But, when the DC voltage applied to the negative terminal of A at any instant is lower than that which is applied to the positive terminal, A generates an alarm signal which is an indication of insufficient oil pressure for a given speed or vice versa. The alarm signal is reflected proportionately to diode D and at the positive terminal of comparator A as an alarm signal, as more fully described below.

The pressure-speed comparator A,- operates in the manner described above because the variable DC voltage output signal from amplifier A of the pressure input circuit indicative of engine oil pressure has been matched to engine specifications as a function of speed represented by the variable DC voltage output signal from the speed input circuit. The matching is done by adjustment to R in the pressure input circuit and by the value selected for R in the speed input circuit.

R sets the bias on comparator A so that the signal applied to the positive terminal of A never goes below a predetermined value, for example 1.8V. Referring to FIG. 4, there is shown the output voltage from R plotted as a function of engine speed. At idle, 500 rpm, the output voltage is 2.0V, fopexample. When the engine is shut down, the voltage, ecreases until it reaches 1.8V

at 400 rpm, and then remains constant at 1.8V until the engine stops. This bias of Vout is set by R,,,.

R unbalances the bridge network of the pressure input circuit so that the output signal ofA applied to negative terminal A never goes below a predetermined value, for example, 2v, which is slightly higher than the bias applied to the positive terminal of A Referring to FIG. 3, there is shown this minimum output voltage level of A which represents zero oil pressure and still prevents A, from tripping at speeds of 400 rpm and lower, a condition existing only during unwinding of the engine after shutdown.

R changes the gain of amplifier A so that the slope of the line representing the variable output voltage of 'A, shown in FIG. 3 can be changed (indicated by dotted lines in FIG. 3) to match the slope of the variable DC output voltage at R shown in FIG. 4. It is readily apparent that under normal operating conditions Vout at A is always slightly higher than Vout at R In FIG. there is shown the curve representing the conditions under which A, will trip. So long as the actual oil pressure remains on or above the minimum oil pressure line, no trip occurs. When the actual oil pressure for a given speed falls below the minimum oil pressure line, A is tripped.

An additional feature of the pressure-speed comparator is a delay circuit which functions to prevent a trip of A, during surges of low pressure or high speed i.e., prevents false trips. The time constant of the delay circuit is defined by R C,,. In operation, when A generates an output signal, C, is charged until its voltage exceeds the forward voltage of D,,- and then the alarm signal is passed to A, in the trip circuit. C,, discharges through R recycling for the next alarm signal. An 8-10 second delay is imposed on the output of A C is a negative feedback capacitor which dampens AC noise vibrations contained in the input signal applied to the negative terminal of A,.

Trip Circuit The purpose of the trip circuit is to detect an alarm signal generated from signals originating in any of the input circuits and trigger the first and second alarm circuits and the alarm output shutdown circuit. The trip circuit includes diodes D,,, D D,,, D, and D,,, resistors R, R R R capacitor C,,, diode D,,, and differential comparator A,. Refer to FIG. 2B.

Diodes D D,, D,,, D, and D are steering diodes which prevent interference between the alarm signals from the high temperature coolant circuit, the openshort sensor circuit, the low coolant level input circuit, the high speed trip circuit and the delayed pressurespeed comparator circuit respectively.

R and R provide biasing for the input terminals of amplifier A,. R, and R constitute a voltage divider setting the reference voltage level of the negative terminal of A,. Whenever the voltage level at the positive terminal of A, is higher than the reference voltage level at the negative terminal, A, senses an alarm condition and the output of A, goes high triggering both alarm circuits and the alarm shutdown circuit simultaneously. C,, is a noise filter whereby noise on one input line is shunted to the other line so that a false alarm is avoided. D, compensates for temperature changes in the steering diodes and follows them.

Alarm Output Circuits When the output of comparator A, goes high, both alarm output circuits are tripped. The A, output signal voltage is used to operate two alarm circuits. One alarm is a 2 ampere load, and the other is a 15 ampere load. The 2 ampere load is usually an audible or visual alarm. i.e., a lamp, a whistle or similar type load, whereas the 15 ampere load is, for example, a solenoid which is used to shut down the engine such as by operating an air valve in the engine intake system. The small load is continuous, whereas the large load is momentary. Refer to FIG. 2B.

The first alarm output circuit includes resistors R R R R and R capacitors C and C, transistor Q SCR 0,, and a 2 amp load at terminal G. In operation, the output of comparator A,, when it goes high, turns on transistor Q through R and R Capacitor C, filters out transient noises to prevent false trips of the alarm. R limits the current flow through Q R sets the source impedence to the gate of SCR 0,. When Q turns on, gating current is supplied to SCR 0,, which is switched on. O is a silicon controlled rectifier, the characteristic of which is that once it is turned on, it re mains on until the whole circuit is de-energized, such as by disconnecting the supply battery. Thus, the 2 amp load connected at terminal G remains in continuous operation until the battery is disconnected.

The second alarm output circuit includes resistors R R R R and R transistors Q Q, and Q,,, diodes D, and D,,, Zener diode Z and a 15 amp load at terminal N. In operation, when the voltage output of comparator A, goes high, O is turned on through R R limits the current flow through Q 0, turns on transistor Q which turns on transistor Q Three transistors are used so that the current level outputs from comparator A, is amplified significantly in order to operate the 15 amp load connected in series with Q,,. When O is turned on, the 15 ampere load is operated. Transistors Q and Q,, are connected to diodes D, and D,,, respectively which protect the transistors against accidental reversal of the battery voltage. Resistors R, and R protect transistors Q and Q respectively, from thermal runaway by drainage leakage current out of the transistors. R limits the base current of Q,,.

There is good reason for only momentarily operating the 15 amp load of the second alarm output circuit. The solenoid and Q and Q draw a lot of power. Should that load stay on for any prolonged period of time, the solenoid would overheat, the transistors would overheat, and the battery would be drained. This would be very undesirable, particularly, for example, in unattended generators. Therefore, an alarm shut down circuit is provided in order to turn off the second alarm output circuit after a few seconds. The alarm shut down circuit includes resistors R R R R R R and R capacitor C,;,, diodes D,,, D,,, and D,,, comparator A and transistor Q,. When the output voltage of comparator A, goes high turning on the 15 amp load alarm as discussed above, current also flows through diode D and R to timing capacitor C,,, which is charged. It takes several seconds to charge C C, is connected to the positive terminal of comparator A,,. R,,, and R are a voltage divider biasing the positive terminal of A,,. The negative terminal of A, has its level set by voltage divider R R Thus, when C goes high, the output of A goes high, and an output signal is generated turning on transistor Q through diode D R,-,,, and R are a voltage divider biasing 0,. When 0,, turns on, it grounds the base terminal of Q turning off which in turn turns offQ and 0,, deactivating the 15 amp load alarm. R ,-,C,;, defines an RC circuit which takes ap proximately 2 to 3 /2 seconds to charge. During that time interval, the second alarm is operative until A is triggered, de-energizing that alarm. The 2 to 3 /2 second interval is sufficient to' shut down the engine. By limiting the heatsinking capacity of the amp output alarm, an additional advantage is obtained in that the entire unit can be made smaller and cheaper. D is a lockout circuit which keeps A on, once it is triggered. until power to the entire circuit is disconnected.

Supply Voltage The supply voltage is provided by a 12, 24 or 36 volt battery at pin F. The entire circuit is designed to work at 10 volts regardless of which size battery is used. Therefore, it is necessary to provide a replaceable dropping resistor R mounted on the outside of the supply box such that a resistor may be selected having either 0, 24 or 47 ohms depending on whether the supply battery is 12, 24 or 36 volts, respectively.

Ten Volt Regulator The voltage regulator circuit comprises D Q R Z and C The operation of the regulator circuit is as follows:

Diode D is connected between the transistor collector and supply and protects against damage to the transistor in the event of accidental reverse battery connection. Zener diode Z is set at a level slightly above the voltage level which it is desired to maintain. in this instance it is set at slightly above 10 volts. Supply voltage is applied to the base of Q, through resistor R turning on When the supply voltage exceeds 12v, Z conducts more, maintaining the base voltage of Q con stant. Setting Z at about 1 lv maintains the voltage output of Q, at 10v. C filters out transient noises. All components of the protection system which require a regulated supply voltage are connected to the plus 10 volt regulator.

We claim:

1. A protection circuit for an internal combustion engine comprising a power source,

means for providing a first input signal which varies as function of a first engine parameter, means for providing a second input signal which varies as a function of a second engine parameter,

means for manually adjusting the second input signal so that selected variations of said second input signal are representative of predetermined variations of said second engine parameter to adapt said protection circuit to said engine,

comparator means for comparing the second input signal to the first input signal to provide an alarm signal in response to a predetermined difference between the two input signals,

means for sensing the alarm signal and for providing an alarm indication, and

power source means connected to said first and second input signal means, tosaid adjusting means, to said comparator means and to said alarm signal sensing means for energization thereof.

2. The protection circuit of claim I wherein the means for sensing the alarm signal and for providing an alarm indication comprises tripping means for generating an output signal in re- 5 sponse'to the alarm signal,

a firstalarm means for continuously indicating the alarm condition'in response to the output signal from the tripping means,

a second alarm means for shutting down the engine in response to the output signal from the tripping means, and

means for de-energizing the second alarm means after a predetermined time interval.

3. The protection circuit of claim 2 further comprising means for delaying the alarm signal from the comparator means a predetermined time interval to prevent false alarms.

4. The protection circuit of claim 1 wherein the first engine parameter is engine speed, the second engine parameter is oil pressure, and the alarm condition is that the actual oil pressure is below a desired level for a given engine speed.

5. The protection circuit of claim 1 wherein the comparator means is an operational amplifier.

6. The protection circuit of claim 1 further comprising means for comparing the first input signal to a predetermined limiting value to provide an alarm signal whenever the predetermined value is exceeded.

7. The protection circuit of claim 6 wherein the predetermined value is representative of a desired maximum speed of the engine and the alarm signal represents an overspeed condition of the engine.

means responsive to a third engine parameter for providing an alarm signal to the tripping means when a predetermined condition is reached.

9. The protection circuit of claim 8 further comprising means responsive to a fourth engine parameter for providing an alarm signal to the tripping means when a predetermined condition is reached.

10. The protection circuit of claim 9 wherein the third and fourth engine parameters are liquid coolant temperature and liquid coolant level, respectively, and the predetermined conditions are liquid coolant temperature above a desired temperature and liquid coolant level below a desired level, respectively.

11. The protection circuit of claim 10 further comprising fail safe means for providing an alarm signal to the tripping means in response to a short or open circuit in either of the means responsive to coolant temperature and level.

12, The protection circuit of claim 11 further comprising means for preventing interference between any of the alarm signals, and wherein the tripping means is responsive to any of the alarm signals.

'13. The protection circuit of claim 9 further comprising fail safe means for providing an alarm signal to the tripping means in response to ashort or open circuit in the means responsive to the fourth engine 8. The protection circuit of claim 6 further compris-' parameter, being liquid coolant level, said fail safe means comprising a primary fail safe circuit which includes an operational amplifier,

a secondary fail safe circuit.

the alarm signal being generated through the primary fail safe circuit by reducing the voltage level at the positive terminal of the amplifier below a predetermined level and reaching the tripping circuit prior to the alarm signal being generated through the secondary fail safe circuit.

14. The protection circuit of claim 9 wherein said power supply means includes variable power supply means connected to said means responsive to the third and fourth engine parameters for energizing said means responsive to said third and fourth engine parameters at a predetermined voltage lower than the voltage applied to energize said first and second input signal means.

15. The protection circuit of claim 1 wherein said power supply means includes means connected to said first and second signal means, to said adjusting means, to said comparator means, and to said alarm signal sensing means for providing a regulated supply voltage thereto.

16. The protection circuit of claim 1 further compris ing means for protecting against accidental reverse application of supply power.

17. The protection circuit of claim 1 wherein the means for adjusting the second input signal in relation to the first input signal includes a resistor which sets the minimum voltage level of the first input signal applied to the comparator means,

a first variable resistor which sets the minimum volt age level of the second input signal applied to the comparator means,

a second variable resistor which sets the rate of change of the voltage level of the second input signal applied to the comparator means.

18. The protection circuit of claim 2 wherein the first alarm means includes a silicon controlled rectifier in series with the load for holding the load in an energized condition until the supply power is disconnected.

19. The protection circuit of claim 2 wherein the means for de-energizing the second alarm includes lockout means for holding the second alarm in deenergized condition until the supply power is disconnected.

20. A protection circuit for an internal combustion engine comprising means for providing a regulated power source,

tripping means for sensing an alarm signal and for generating an output signal in response thereto,

alarm means for providing an alarm indication in response to an output signal from the tripping means,

first input means responsive to a first engine parameter for providing an alarm signal to the tripping means when a predetermined condition is reached,

second input means responsive to a second engine parameter for providing an alarm signal to the tripping means when a predetermined condition is reached, and

fail safe means for providing an alarm signal to the tripping means in response to a short or open cir cuit in either the first input means or the second input means, said tripping means, alarm means, said first and second input means and said fail safe means being connected to said power source for receiving power therefrom.

21. The protection circuit of claim 20 wherein the alarm means comprises a first alarm means for continuously indicating the alarm condition in response to the output signal from the tripping means.

a second alarm means for shutting down the engine in response to the output signal from the tripping means, and

means for deenergizing the second alarm means after a predetermined time interval.

22. The protection circuit of claim 20 further comprising means for preventing interference between any of the alarm signals, and wherein the tripping means is responsive to any of the alarm signals.

23. The protection circuit of claim 20 wherein the first and second engine parameters are liquid coolant temperature and liquid coolant level, respectively, and the predetermined conditions are liquid coolant temperature above a desired temperature and liquid coolant level below a desired level, respectively.

24. The protection circuit of claim 20 wherein the fail safe means for providing an alarm signal to the trip ping means in response to a short or open circuit in the second input means comprises a primary fail safe circuit which includes an operational amplifier in addition to the tripping means,

a secondary fail safe circuit,

the alarm signal being generated through the primary fail safe circuit by reducing the voltage level at the positive terminal of the amplifier below a predetermined level and reaching the tripping means prior to the arrival at the tripping means of the alarm signal being generated through the secondary fail safe circuit.

25. The protection circuit of claim 24 wherein the second engine parameter is liquid coolant level.

26. The protection circuit of claim 25 wherein the primary fail safe circuit for providing an alarm signal in the event of an open or short circuit in the liquid coolant level sensing means is the same circuit which provides the alarm signal in the event of low liquid coolant level.

27. The protection circuit of claim 24 wherein the alarm signal being provided by the secondary fail safe circuit is a significantly reduced reference voltage level for the tripping means.

28. The protection circuit of claim 24 wherein the secondary fail safe circuit includes means to reduce the base voltage level of a normally conducting semiconductor which reduces its voltage output level which is reflected proportionately at a reference terminal in the tripping means. 

1. A protection circuit for an internal combustion engine comprising a power source, , means for providing a first input signal which varies as function of a first engine parameter, means for providing a second input signal which varies as a function of a second engine parameter, means for manually adjusting the second input signal so that selected variations of said second input signal are representative of predetermined variations of said second engine parameter to adapt said protection circuit to said engine, comparator means for comparing the second input signal to the first input signal to provide an alarm signal in response to a predetermined difference between the two input signals, means for sensing the alarm signal and for providing an alarm indication, and power source means connected to said first and second input signal means, to said adjusting means, to said comparator means and to said alarm signal sensing means for energization thereof.
 2. The protection circuit of claim 1 wherein the means for sensing the alarm signal and for providing an alarm indication comprises tripping means fOr generating an output signal in response to the alarm signal, a first alarm means for continuously indicating the alarm condition in response to the output signal from the tripping means, a second alarm means for shutting down the engine in response to the output signal from the tripping means, and means for de-energizing the second alarm means after a predetermined time interval.
 3. The protection circuit of claim 2 further comprising means for delaying the alarm signal from the comparator means a predetermined time interval to prevent false alarms.
 4. The protection circuit of claim 1 wherein the first engine parameter is engine speed, the second engine parameter is oil pressure, and the alarm condition is that the actual oil pressure is below a desired level for a given engine speed.
 5. The protection circuit of claim 1 wherein the comparator means is an operational amplifier.
 6. The protection circuit of claim 1 further comprising means for comparing the first input signal to a predetermined limiting value to provide an alarm signal whenever the predetermined value is exceeded.
 7. The protection circuit of claim 6 wherein the predetermined value is representative of a desired maximum speed of the engine and the alarm signal represents an overspeed condition of the engine.
 8. The protection circuit of claim 6 further comprising means responsive to a third engine parameter for providing an alarm signal to the tripping means when a predetermined condition is reached.
 9. The protection circuit of claim 8 further comprising means responsive to a fourth engine parameter for providing an alarm signal to the tripping means when a predetermined condition is reached.
 10. The protection circuit of claim 9 wherein the third and fourth engine parameters are liquid coolant temperature and liquid coolant level, respectively, and the predetermined conditions are liquid coolant temperature above a desired temperature and liquid coolant level below a desired level, respectively.
 11. The protection circuit of claim 10 further comprising fail safe means for providing an alarm signal to the tripping means in response to a short or open circuit in either of the means responsive to coolant temperature and level.
 12. The protection circuit of claim 11 further comprising means for preventing interference between any of the alarm signals, and wherein the tripping means is responsive to any of the alarm signals.
 13. The protection circuit of claim 9 further comprising fail safe means for providing an alarm signal to the tripping means in response to a short or open circuit in the means responsive to the fourth engine parameter, being liquid coolant level, said fail safe means comprising a primary fail safe circuit which includes an operational amplifier, a secondary fail safe circuit, the alarm signal being generated through the primary fail safe circuit by reducing the voltage level at the positive terminal of the amplifier below a predetermined level and reaching the tripping circuit prior to the alarm signal being generated through the secondary fail safe circuit.
 14. The protection circuit of claim 9 wherein said power supply means includes variable power supply means connected to said means responsive to the third and fourth engine parameters for energizing said means responsive to said third and fourth engine parameters at a predetermined voltage lower than the voltage applied to energize said first and second input signal means.
 15. The protection circuit of claim 1 wherein said power supply means includes means connected to said first and second signal means, to said adjusting means, to said comparator means, and to said alarm signal sensing means for providing a regulated supply voltage thereto.
 16. The protection circuit of claim 1 further comprising means for protecting against accidental reverse application of supply power.
 17. The protection circuit of claim 1 Wherein the means for adjusting the second input signal in relation to the first input signal includes a resistor which sets the minimum voltage level of the first input signal applied to the comparator means, a first variable resistor which sets the minimum voltage level of the second input signal applied to the comparator means, a second variable resistor which sets the rate of change of the voltage level of the second input signal applied to the comparator means.
 18. The protection circuit of claim 2 wherein the first alarm means includes a silicon controlled rectifier in series with the load for holding the load in an energized condition until the supply power is disconnected.
 19. The protection circuit of claim 2 wherein the means for de-energizing the second alarm includes lockout means for holding the second alarm in de-energized condition until the supply power is disconnected.
 20. A protection circuit for an internal combustion engine comprising means for providing a regulated power source, tripping means for sensing an alarm signal and for generating an output signal in response thereto, alarm means for providing an alarm indication in response to an output signal from the tripping means, first input means responsive to a first engine parameter for providing an alarm signal to the tripping means when a predetermined condition is reached, second input means responsive to a second engine parameter for providing an alarm signal to the tripping means when a predetermined condition is reached, and fail safe means for providing an alarm signal to the tripping means in response to a short or open circuit in either the first input means or the second input means, said tripping means, alarm means, said first and second input means and said fail safe means being connected to said power source for receiving power therefrom.
 21. The protection circuit of claim 20 wherein the alarm means comprises a first alarm means for continuously indicating the alarm condition in response to the output signal from the tripping means, a second alarm means for shutting down the engine in response to the output signal from the tripping means, and means for de-energizing the second alarm means after a predetermined time interval.
 22. The protection circuit of claim 20 further comprising means for preventing interference between any of the alarm signals, and wherein the tripping means is responsive to any of the alarm signals.
 23. The protection circuit of claim 20 wherein the first and second engine parameters are liquid coolant temperature and liquid coolant level, respectively, and the predetermined conditions are liquid coolant temperature above a desired temperature and liquid coolant level below a desired level, respectively.
 24. The protection circuit of claim 20 wherein the fail safe means for providing an alarm signal to the tripping means in response to a short or open circuit in the second input means comprises a primary fail safe circuit which includes an operational amplifier in addition to the tripping means, a secondary fail safe circuit, the alarm signal being generated through the primary fail safe circuit by reducing the voltage level at the positive terminal of the amplifier below a predetermined level and reaching the tripping means prior to the arrival at the tripping means of the alarm signal being generated through the secondary fail safe circuit.
 25. The protection circuit of claim 24 wherein the second engine parameter is liquid coolant level.
 26. The protection circuit of claim 25 wherein the primary fail safe circuit for providing an alarm signal in the event of an open or short circuit in the liquid coolant level sensing means is the same circuit which provides the alarm signal in the event of low liquid coolant level.
 27. The protection circuit of claim 24 wherein the alarm signal being provided by the secondary fail safe circuit is a Significantly reduced reference voltage level for the tripping means.
 28. The protection circuit of claim 24 wherein the secondary fail safe circuit includes means to reduce the base voltage level of a normally conducting semiconductor which reduces its voltage output level which is reflected proportionately at a reference terminal in the tripping means. 